Method and apparatus for replacing lost PSTN data in a packet network

ABSTRACT

A method and apparatus for replacing lost PSTN data in a packet network and for generating variable power white noise are disclosed. In one embodiment, the method for replacing data includes the steps of receiving data packets, detecting lost data packets, and producing in response a lost data output indicating when replacement data needs to be provided. The method preferably generates the replacement data by re-using data stored in an extended playback buffer (with the re-used replacement data starting with the oldest output data byte stored in the extended playback buffer), and placing the re-used replacement data on an external network. Another aspect of the invention is a variable power white noise generator for providing replacement data. The variable power white noise generator may use, for example, a multi-bit register that stores a magnitude, and a pseudo-random sign bit generator to change the positive and negative sense of the magnitude. In one preferred embodiment, a linear feedback shift register (LFSR) is used in conjunction with a feedback network corresponding to a polynomial generating function. The pseudo-random output sequence of the LFSR then produces a pseudo-random output bit to control the positive and negative sense of the magnitude bits.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to replacement of data lost or corruptedin a packet network. In particular, the present invention relates to amethod and apparatus using selectable replacement data generators toreplace lost PSTN data in a packet network.

Modern communications networks carry staggering amounts of datagenerated by numerous and diverse information sources. The PublicSwitched Telephone Network (PSTN) (which is a Time Division Multiplex(TDM) communications network) carries, for example, digital datarepresenting voice signals and digital data produced by faxes andMODEMs. Furthermore, the data may travel over any number of variednetworks in route from the information source to the informationdestination, with each network incorporating divergent topologies androuting protocols.

Some networks divide data traffic into discrete packets which arereferred to as “frames”, “packets”, or “cells”, depending on theparticular network. As an example, an Asynchronous Transfer Mode (ATM)network is one of the many network solutions currently available. An ATMnetwork supports high speed data transfers by dividing the data intoindividual “cells”. An ATM network cell is 53 bytes long and includes 48information bytes and 5 network control bytes. It is often possible tolink two or more dissimilar networks with an interworking device.

For example, an ATM network may accept, through an interworking device,a synchronous byte stream of digitized voice or MODEM data from a localPSTN or Public Branch Exchange (PBX) that may, for example, connectdirectly to a local loop and home telephone lines. The ATM networkgroups these bytes into cells, and routes the packetized data over highspeed ATM links to a destination. At the destination, the ATM networkinterfaces, for example, with a PSTN through an interworking device. Theinterworking device accepts the asynchronous stream of packets andconverts this data to a synchronous stream of bytes to be delivered tothe PSTN network. An ATM to PBX interworking function may be provided,for example, by a Tellabs AN2100 multiplexer, available from Tellabs,Inc., Lisle Ill.

Many network protocols include provisions for determining when cells orpackets are lost or corrupted. In ATM networks that carry cells createdby the ATM Adaptation Layer 1 protocol (AAL1), for example, one of theinformation bytes is an information control byte that includes a 3-bitsequence number that is incremented each time a new cell is sent overthe network. A receiver may then detect a missing cell by monitoring thesequence number. For example, receiving a sequence of cells 1,2,4,5reveals that cell 3 has been lost.

In order to maintain bit count integrity in the PSTN network, asubstitution function in the interworking device at the ATM networkdestination typically inserts dummy data in place of the missing cell.The amount of dummy data inserted is equal to the amount of data carriedby the lost cell, for example, an ATM cell using AALI may carry 47 bytesof data. The substitution function typically takes one of threeapproaches to generating dummy data: inserting silence, inserting whitenoise, or inserting (repeating) previous data. The type of dummy datathat the substitution function inserts is of particular importance whenthe dummy data is communicated out to the PSTN (and connected phonelines, fax machines, and MODEMs).

Inserting silence may be implemented by using a single, constant valueas the dummy data for each byte in a lost or corrupted cell. The silenceresults because a series of constant values has a predominately DC(i.e., zero frequency) component. The DC component is filtered out bythe CODECs (which typically roll off in frequency response below 300 Hzand above 3400 Hz, with severe attenuation at approximately 0 Hz or DC)that convert the PSTN data to analog form for reproduction at atelephone receiver.

Inserting silence often has undesirable effects when the series ofconstant values makes its way back out to the PSTN, however. Forinstance, if the dummy data forms part of a digitized voiceconversation, the constant values manifests themselves as completesilence at a PSTN receiver. Complete silence makes a telephone callsound as if the line has gone dead or has otherwise been disconnected.Furthermore, if the dummy data forms part of a voiceband data connection(for a fax machine, for example), then complete silence may beinterpreted as a disconnection or loss of carrier by a receiving faxmachine. Thus a MODEM or fax machine may “drop out”, i.e., drop aconnection, or otherwise lose the ability to transmit or receive data onthe connection when it receives inserted silence.

In the past, one method for repeating old data simply replayed a singlepiece of old data, for example, the last cell successfully received.Inserting a single cell of old data may cause audible false tones orfrequency components, particularly if more than a few sequential cellsare lost and the same old cell is repeated more than a few times.

Inserting white noise is a third option available to a substitutionfunction. A generally accepted definition of white noise is that a whitenoise signal consists of samples whose values are uncorrelated with oneanother. In other words, the given sample value cannot be predicted withany additional accuracy even given a complete knowledge of all past andfuture samples. Noise substitution in general has been proposed, forexample, in the American Nation Standards Institute (ANSI) T1 standardT1.312 of 1991. The T1 standard specifies a set of 16 noise power levelsthat should be used, but does not provide any method for generatingnoise of any type.

White noise substitution provides a sound similar to the backgroundstatic sound familiar from periods of silence during phone conversationsor loss of radio or television reception. The static sound reinforcesthe perception during a phone call, for example, that the connection isstill intact (that the line has not gone dead). Ideal white noiseproduces energy in all frequency bands and MODEM and fax machinestypically interpret a received signal with energy at a particularfrequency with a predetermined range as a live connection. Thus,regardless of the particular frequency or frequency band in which aMODEM expects to find energy representing a live connection, that energymay be provided by white noise substitution. Inserting white noise withmore than the threshold amount of energy therefore prevents, in manyinstances, MODEMs and fax machines from terminating a connection.

One past approach to generating white noise has centered aroundreal-time speech coding. In J. F. Lynch Jr. et al., Speech/SilenceSegmentation for Real-Time Coding Via Rule Based Adaptive EndpointDetection, ICASSP '87, 1348-1351 (1987) (the “ICASSP paper”), theauthors propose a general speech coding scheme. The coding schemeincludes a receiver that reconstructs speech by decoding speech segmentsand that reconstructs silence with a noise substitution processcontrolled through amplitude and duration parameters. The noisesubstitution process in the ICASSP paper uses a 18th order polynomial togenerate a spectrally flat pseudo-random bit sequence that is filteredto match the mean coloration of acoustical background noise.

The ICASSP paper proposes a set of production rules that generate asingle scalar quantity called a noise metric that indicates the currentlevel of background noise. At the receiver, a binary shift registerproduces a binary sequence that is modulated with the noise metric toproduce noise at the amplitude of the original background noise. Inaddition, the modulated output is passed through a low pass filter toshape the simulated noise to approximate the average “coloration” of thebackground noise. The approach in the ICASSP paper thus does not providean independent mechanism for manipulation of the output noise powerlevel. Rather, the output noise amplitude is responsive to a mechanismthat is required to generate a noise metric. Furthermore, the ICASSPpaper requires an output low pass filter to further shape the outputnoise appropriately for merging with decoded speech segments.

A need exists in the industry for a method and apparatus for replacinglost PSTN data in a packet network, including flexible and efficientmechanisms for lost data substitution and white noise generation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for replacing lost PSTN data in a packet network.

It is another object of the present invention to provide a lost datasubstitution method.

Yet another object of the present invention is to provide a lost datasubstitution method that uses an extended data buffer which providesreplacement data and that minimizes false tones or spurious frequencycomponents in the data communicated to the PSTN.

It is an object of the present invention to provide a variable powerwhite noise generator for use with modern networking components.

It is another object of the present invention to provide a variablepower white noise generator that provides direct control over the whitenoise output power.

A further object of the present invention is to use a linear feedbackshift register in a lost data substitution device to provide apseudo-random white noise output.

Another object of the present invention is to generate white noise at apower level suitable to maintain fax, MODEM, and data connections.

Another object of the present invention is to generate white noise at apower level suitable to indicate a live telephone connection.

A still further object of the present invention is to provide aninterworking unit including multiple distinct data replacementgenerators, each of which may be selected according to a replacementdata selection input.

The present invention provides a method and apparatus for replacing lostPSTN data in a packet network and for generating variable power whitenoise. In one embodiment, the method for replacing data includes thesteps of receiving data packets, detecting lost data packets, andproducing in response a lost data output indicating when replacementdata needs to be provided.

The method may then generate replacement data by re-using data stored inan extended playback buffer (with the re-used replacement data startingwith the oldest output data byte stored in the extended playbackbuffer), and placing the re-used replacement data on an external networkwhen the lost data output indicates replacement data needs to beprovided. Alternatively, other replacement data generators may providethe replacement data. For example, a variable power white noisegenerator or a silence generator may provide the replacement data.

Another aspect of the invention is a variable power white noisegenerator including, for example, a multi-bit register storing amagnitude, and a pseudo-random sign bit generator for changing thepositive and negative sense of the magnitude. As one example, themulti-bit register may be implemented as an 8-bit register with one signbit and seven magnitude bits to represent values from −127 to +127.Alternately, larger or smaller registers may be used as well asalternate predetermined number formats, for example, two's complementrepresentation.

In one preferred embodiment, the variable power white noise generatoruses a linear feedback shift register (LFSR) including n bit positions.The LFSR is connected to a feedback network which corresponds to apolynomial generating function. The generating function preferablyproduces a pseudo-random output sequence of length 2^(n)−1. Thepseudo-random output sequence of the LFSR may then provide thepseudo-random sign bit which controls the positive and negative sense ofthe magnitude bits. For example, the sign bit may be placed in a signbit location in the multi-bit register or otherwise combined with themagnitude in downstream processing.

The output of the multi-bit register provides a white noise output ofvariable power by changing the value stored in the magnitude bits. Thewhite noise output may then be used, for example, to fill in missingdata in an output data stream and the power may be adjusted to apredetermined level for voice conversations and a second predeterminedlevel for data communications, or for finer control, the power level maybe determined adaptively for a particular connection. Severalindependent multi-bit registers or LFSRs may be provided to handlenumerous independent output data streams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of a Linear Feedback Shift Register(LFSR) suitable for use in a variable power white noise generator.

FIG. 2 shows an LFSR and multi-bit output register used to provide avariable power white noise output.

FIG. 3 shows one example of a bank of white noise generators used toprovide independent variable power white noise outputs for multipleindependent output data streams. Note that only one LFSR is needed forthe entire bank of white noise generators.

FIG. 4 shows a block diagram of an interworking unit including lostpacket detection and replacement data generation functions.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 4, that figure shows a block diagram of aninterworking function 400. The interworking function 400 includes a lostpacket detection unit 402 connected to a data processing unit 404. Alsoincluded is a replacement data unit 406 connected to a data playout unit408. It is noted that one or more of the interworking blocks 402-408shown in FIG. 4 may be implemented in a single ASIC, PLD, or the like,or alternatively with discrete circuitry, or software.

The lost packet detection unit 402 receives a packet stream S_(i),(i.e., packet data for connection i), as an input. The lost packetdetection unit 402 examines each packet in the stream S_(i) to determinewhen packets are missing (for example, by checking sequence numbers),corrupted (for example, by checking error correcting codes), notavailable when needed (for example, by checking a received data queuesize), or otherwise require substitution with replacement data. The lostpacket unit 402 indicates when replacement data is needed by assertingor deasserting one or more signals L_(i).

The packet stream S_(i) is also connected to the data processing unit404. In the data processing unit 404, packet overhead bytes are removedand the remaining data, D_(i), is processed and forwarded to the dataplayout unit 408. The overhead bytes typically represent informationsuch as sequence numbers (in ATM networks in particular), but may alsoinclude any overhead information added to a packet including errorcorrecting codes, source/destination routing information, and the like.

When replacement data is not needed (as indicated by L_(i)), the dataplayout unit 408 places the data D_(i) on the PSTN in particular timeslots according to PSTN standards. It is noted, however, that thepresent invention is not limited to interworking with the PSTN. Rather,the interworking device 400 and data playout unit 408 may connect tovirtually any destination network provided that the data playout unit408 appropriately formats the data according to the standards for thatdestination network. When L_(i) indicates that replacement data isneeded, however, the data playout unit 408 places the replacement dataR_(i) on the PSTN instead of the data Di. The replacement data R_(i) maybe provided by the data replacement unit 406 or the data playout unit408.

The data replacement unit 406 or the data playout unit 408 may includeseveral distinct replacement data generators. For example, onereplacement data generator may be implemented as an extended playbackbuffer, another replacement data generator may be implemented as avariable power white noise generator, and yet another replacement datagenerator may be a silence generator (each described in more detailbelow). The data replacement unit 406 may determine which replacementdata generator to use based upon the T_(i) input. An echo canceller mayprovide T_(i) to indicate that connection i is a data connection (i.e.,fax or MODEM), a voice connection, or another type of connection.Another example source for T_(i) is an SS7 unit.

As noted above, one of the distinct replacement data generators may beimplemented as a variable power white noise generator, described indetail with reference to FIGS. 1 and 2. Turning first to FIG. 1, thatfigure shows a Linear Feedback Shift Register (LFSR) generally indicated100. The LFSR 100 includes a feedback network 102 which corresponds to agenerator polynomial and a series of registers 104 (register 0 throughregister 17). The feedback network 102 in FIG. 1 uses a summation node106 that adds the contents of register 0 and register 7 and places theresult in register 17. The bit stored in register 0 is used as theoutput bit 108.

The LFSR 100 in FIG. 1 includes 18 registers 104 and implements thegenerator polynomial X¹⁸+X⁷+1. In general, any generator polynomial ofthe form G(X)=g_(m)X^(m)+g_(m-1)X^(m-1)+ . . . +g₁X+g₀ may beimplemented using additional summation nodes. As will be explainedbelow, however, primitive polynomials are preferred. The LFSR 100produces two output sequences. One sequence, of period one, is theoutput sequence of all zeros generated when the initial state of each ofthe registers 104 is zero. The other sequence produced by the LFSR 100(generated from any starting condition other than all zeros) is ofperiod 2^(n)−1, where n is the number of registers 104.

The LFSR 100 and associated feedback network 102 produce a pseudo-randomoutput of length 2^(n)−1. Each new output is generated by evaluating thenew contents for register 17 and shifting the contents of the registers104 to the left. Because the LFSR 100 output will be used to fill cellsof missing data which may represent telephone conversation samples, thelength 2^(n)−1 is preferably long enough to prevent the sequence fromrepeating at a rate that creates an audible frequency component in thetelephone conversation. The rate of repetition, in turn, dependsunpredictably on the number of cells or frames lost over a network. Thevalue for n is therefore typically chosen heuristically. For example, avalue for n of 18 is typically acceptable, though much smaller valuesmay be used in networks less susceptible to lost frames and cells.

Turning now to FIG. 2, that figure shows the LFSR 100 connected to amulti-bit register 200. The multi-bit register 200 is shown includingeight bits: a sign bit 202 and seven magnitude bits 204. The magnitudebits 204 may be connected to any general purpose data bus and providedwith values between 0 and 127. If additional levels of magnitude arerequired, the multi-bit register 200 may be extended with additionalmagnitude bits. Similarly, the multi-bit register 200 may be shortenedto reduce the number of magnitudes available. The multi-bit register 200may store a sign magnitude (one's complement) binary value, for example,or may store a binary value in another predetermined format, includingtwo's complement, A-law, or μ-law.

The output bit 108 controls whether the magnitude represented by themagnitude bits 204 will be stored as a positive or negative (signed)quantity. Because the output bit 108 is generated by the pseudo-randomoutput of the LFSR 100, the signed quantity fluctuates between positiveand negative values in a pseudo-random, uncorrelated fashion. Thus, thecontents of register 200 produce a pseudo-white noise signal whose powermay be varied by modifying the contents of the magnitude bits 204. Asdescribed below, data connections (i.e., for faxes and MODEMs) arepreferably maintained using an extended playback buffer. However, themagnitude bits 204 may be adjusted in accordance with L_(i), forexample, to provide a low level background white noise for a telephoneconversation and to provide a high level background white noise toprevent a fax or MODEM from disconnecting a data connection.

For example, in the logarithmic μ-law representation that is used inNorth America, the samples placed on the PSTN are 8 bit samples. Whenusing a fixed magnitude value and randomly alternating the sign bit, themaximum power defined for this signal is approximately +3 dBm0, and theminimum non-zero power is approximately −69 dBm0. Typical fax and modemequipment determines that a signal power above approximately −42 dBm0represents a live connection. The data representing a voice calltypically resides at approximately −15 to −20 dBm0 and has a noiseenergy level below −40 dBm0. Thus, the magnitude bits 204 may be setwithin a range between −69 dBm0 and −40 dBm0 for a voice call. For adata call, the magnitude bits 204 may be set above a predeterminedthreshold, for example, −42 dBm0, or within a range, for example between−42 dBm0 and 0 dBm0.

As noted above, one of the distinct replacement data generators may beimplemented as a silence generator. One efficient mechanism forgenerating silence uses the structure of the variable power white noisegenerator described above. In order to generate silence, the datareplacement unit 406 ignores, makes constant, or provides a constantsign bit 202 with the magnitude bits 204. The result is a constantoutput value which depends on the magnitude bits 204. Silence resultsbecause a series of constant values has a predominately DC (i.e., zerofrequency) component. The DC component is filtered out by the CODECs(which typically roll off in frequency response below 300 Hz and above3400 Hz, with severe attenuation at approximately 0 Hz or DC) thatconvert the PSTN data to analog form for reproduction at a telephonereceiver.

Yet another alternative replacement data generator may be implemented asan extended playback buffer. The playback buffer may store, for example,512 bytes of previously correctly received packet data. The playbackbuffer may be flexibly located, and may be placed, for example, in thedata replacement unit 406 or the data playout unit 408. Preferably, theplayback buffer is located in the playout unit 408 because the extracteddata D_(i) to be stored in the playback buffer passes through theplayout unit 408 in normal operation. Extra connections are thereforeeliminated, for example, between the data extraction unit 404 and thereplacement data unit 406.

The L_(i) signal indicates when replacement data is to be provided bythe extended playback buffer. As an example, the extended playbackbuffer may be used for data connections including fax and MODEMconnections for example, as determined by an echo canceller. Inresponse, the extended playback buffer preferably provides replacementdata starting with the oldest data packet stored in the playback bufferand proceeding to the most recently stored data packet. Thus, there isno repetition of replacement data unless more bytes are lost than can bestored in the playback buffer. An extended playback buffer, for example,512 bytes in length provides a much richer frequency representation(approximating the frequency content of the original signal) than pastschemes which repeat a much smaller buffer, for example, one packet,over and over. The extended playback buffer also reduces the probabilityof introducing false frequencies for even extended losses of data.

Turning to FIG. 3, that figure shows one configuration 300 of variablepower white noise generators used to provide replacement data formultiple independent connections. The configuration 300 includes aseries of multi-bit registers 304 connected to an LFSR 306, a registercontrol 308, and a multiplexer 310. The interworking components shown inFIG. 3 may, for example, be included in the data replacement unit 406.

As noted above, an interworking function provides a mechanism by whichdata is delivered to the destination PSTN from a dissimilar network (forexample, an ATM network). As one example, the interworking function 400may interface to a destination PSTN T1 line that carries multiple voiceor data connections using 24 Time Division Multiplex time slots. Becausedata may be lost in the ATM network for any conversation that will berouted to the destination PSTN, the interworking function 400 mayinclude a series of multi-bit registers 304 that replace the missingdata for any time slot with white noise of power appropriate for theapplication using that time slot. Furthermore, the power output of thewhite noise generated by each multi-bit register 304 may be adjustedindependently. Although FIG. 3 specifically shows an example of multiplevariable power white noise generators, it is noted that multiple copiesof each replacement data generator may be provided additionally oralternatively.

Thus, each register 304 may hold replacement data for a connection. Thepower level stored in each register 304 is preferably set for theduration of the connection. Additionally, each register, and thereforeeach connection, may be set to an independent power level by modifyingthe magnitude bits in each register.

A single LFSR 306 may provide the pseudo-random output controlling thepositive and negative sense of each signed quantity stored in each ofthe multi-bit registers 304. Alternately, additional independent LFSRsmay be connected to subsets of the multi-bit registers 304. In eachinstance where a frame or cell of data is missing, the multi-bitregister 304 assigned to the outgoing time slot (coordinated by themultiplexer 310) may be used to generate white noise samples to replacethe data in the missing frame or cell. The LFSR 306 may then be shifted(once for every byte of replacement data output) to produce a newpseudo-random output stored in the sign bit of each register 304.Missing cells are detected by the lost packet detection unit 402. Thelost packet detection unit 402, as noted above, may function bymonitoring sequence numbers of incoming frames or cells and detecting amissing sequence number or by other means.

The register controller 308 independently adjusts the magnitude portionsof the multi-bit registers 304 and may function under generalmicro-controller and programmed control. As noted above, the magnitudeof the multi-bit registers 304 may be adjusted, for example, dependingon the desired output power level of the white noise. Adjustmenttypically occurs at call setup. One power level may be used for fax andMODEM calls, for example, while another power level may be used forvoice data. Adjustment may occur after call setup, for example, based onmeasured signal values.

The determination of call type (e.g., fax or voice) may come from headerand control information provided in previous frames or cells of data. Inmany instances, however, an echo canceller will be installed in or withthe interworking equipment 300. The echo canceller very closely monitorsthe call data to determine the type of call and corresponding echocancellation properties. Preferably, therefore, the echo cancellerprovides an output that indicates the call type during its normal courseof operation.

A variety of techniques may be used to implement the LFSR 100 andmulti-bit register 200. For example, flip flops, SRAM cells, or DRAMcells may form the basic building blocks for the LFSR 100 and multi-bitregister 200. In addition, discrete logic, PLDs, or custom VLSI designsmay be used to create the white noise generator of the presentinvention, including multiple LFSRs and multi-bit registers.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it is understood that theinvention is not limited thereto since modifications may be made bythose skilled in the art, particularly in light of the foregoingteaching. It is therefore contemplated by the appended claims to coversuch modifications as incorporate those features which come within thespirit and scope of the invention.

What is claimed is:
 1. A method for generating variable power whitenoise for data replacement in a packet network, the method comprising:storing a selected magnitude in a multi-bit register; generating apseudo-random sign bit using a linear feedback shift register; andproviding the sign bit in conjunction with the magnitude as apseudo-random variable power white noise output output value.
 2. Themethod of claim 1, wherein the step of generating comprises evaluatingregister contents of the linear feed back shift register in a feedbacknetwork and shifting the contents of the linear feedback shift register.3. The method of claim 2, wherein the step of evaluating registercontents of the linear feed back shift register in a feedback networkcomprises evaluating register contents of the linear feed back shiftregister in a primitive polynomial feedback network.
 4. The method ofclaim 1, wherein the step of storing a selected magnitude comprisesstoring a selected magnitude corresponding to an absolute energy levelabove a predetermined threshold for a data call.
 5. The method of claim4, further comprising the step of outputting the output value onto adestination network.
 6. The method of claim 1, wherein the step ofstoring a selected magnitude comprises storing a selected magnitudecorresponding to an absolute energy level within a predetermined rangefor a data call.
 7. The method of claim 1, wherein the step of storing aselected magnitude comprises storing a selected magnitude correspondingto an absolute energy level within a predetermined range for a voicecall.
 8. The method of claim 7, further comprising the step ofoutputting the output value onto a destination network.
 9. The method ofclaim 1, further comprising the step of outputting the output value ontoa destination network.
 10. A variable power white noise generatorcomprising: a multi-bit register storing a signed quantity in apredetermined format representing a magnitude with a sign; a linearfeedback shift register connected to a sign bit in the multi-bitregister, the linear feedback shift register including an output bitdetermining the sign of the magnitude; a feedback network connected tothe linear feedback shift register, the feedback network implementing agenerator function; and a register control coupled to the multi-bitregister that stores the magnitude in the multi-bit register.
 11. Thevariable power white noise generator of claim 10, further comprising alost packet detection unit including a lost data output.
 12. Thevariable power white noise generator of claim 10, further comprising atleast one additional multi-bit register storing a signed quantity in apredetermined format representing a magnitude with a sign, and whereinthe linear feedback shift register is further connected to the sign bitof the additional multi-bit register to control the sign of themagnitude stored in the additional multi-bit register.
 13. The variablepower white noise generator of claim 12, further comprising amultiplexer for sequentially providing as output the signed quantity ofeach of the multi-bit registers.
 14. The variable power white noisegenerator of claim 12, wherein the register control is coupled to eachof the multi-bit registers for independently storing magnitudes therein.15. The variable power white noise generator of claim 10, wherein themagnitude is greater than a predetermined data call threshold.
 16. Thevariable power white noise generator of claim 15, wherein the data callthreshold is approximately −42 dBm0.
 17. The variable power white noisegenerator of claim 10, wherein the magnitude is greater than apredetermined voice call threshold.
 18. The variable power white noisegenerator of claim 17, wherein the voice call threshold is greater than−69 dBm0 and the magnitude corresponds to less than −40 dBm0.
 19. Amethod for replacing lost PSTN data in a packet network, the methodcomprising the steps of: receiving data packets from the packet network;detecting lost data packets and producing in response a lost data outputindicating when replacement data needs to be provided; removing overheadinformation from the data packets to produce output data; storing aplurality of output data in an extended playback buffer; generatingduplicate replacement data using the extended playback buffer, theduplicate replacement data starting with the oldest output data storedin the extended playback buffer; placing the duplicate replacement dataon an external network when the lost data output indicates replacementdata needs to be provided and placing the output data on the externalnetwork otherwise.
 20. The method of claim 19, further comprising thesteps of generating white noise replacement data using at least onevariable power white noise generator, each variable power white noisegenerator producing the white noise replacement data by the steps of:using a linear feedback shift register to control a sign bit of amulti-bit register; storing a magnitude in the multi-bit registeroutputting the sign bit and magnitude as the white noise replacementdata; and shifting contents of the linear feedback shift register; andwherein placing comprises placing on the external network one of thewhite noise replacement data and duplicate replacement data according toa replacement data selection signal.
 21. The method of claim 20, furthercomprising the step of generating silence replacement data and whereinthe placing step responds to the replacement data selection signal byplacing white noise replacement data, duplicate replacement data, orsilence replacement data on the external network.
 22. The method ofclaim 20, wherein the magnitude is greater than a predetermined datacall threshold.
 23. The method of claim 22, wherein the data callthreshold is approximately −42 dBm0.
 24. The method of claim 20, whereinthe magnitude is greater than a predetermined voice call threshold. 25.The method of claim 24, wherein the voice call threshold is greater than−69 dBm0 and the magnitude corresponds to less than −40 dBm0.
 26. Amethod for replacing lost PSTN packet data in a packet network, themethod comprising the steps of: receiving data packets from the packetnetwork; detecting lost data packets and producing in response a lostdata output indicating when replacement data needs to be provided;removing overhead information from the data packets to produce outputdata; generating white noise replacement data using a variable powerwhite noise generator producing the white noise replacement data by thesteps of: using a linear feedback shift register to control a sign bitof a multi-bit register; storing a magnitude in the multi-bit register;outputting the sign bit and magnitude as the white noise replacementdata; and shifting the contents of the linear feedback shift register;and placing the white noise replacement data on an external network whenthe lost data output indicates replacement data need to be provided andplacing the output data on the external network otherwise.
 27. Themethod of claim 26, further comprising the step of generating silencereplacement data and wherein placing comprises placing one of thesilence replacement data and the white noise replacement data on theexternal network in accordance with the replacement data selectionsignal.
 28. The method of claim 26, wherein the magnitude is greaterthan a predetermined data call threshold.
 29. The method of claim 26,wherein the magnitude is greater than a predetermined voice callthreshold.
 30. The method of claim 26, further comprising the step ofmultiplexing white noise replacement data from a plurality of variablepower white noise generators onto the external network.
 31. The methodof claim 30, further comprising the step of using the linear feedbackshift register to control a sign bit in each of the plurality ofvariable power white noise generators.
 32. The method of claim 30,further comprising the step of independently storing magnitudes in eachof the plurality of variable power white noise generators according to acall type for which each of the plurality of variable power white noisegenerators provides replacement data.